Theory and Stuff, yet again….
THE STEAM
POWER
CYCLE,
a brief
overview.
EXPANSION in terms of the Rankine Cycle is the process whereby steam expanding to lower
temperature and pressure exerts force against a piston or turbine blade which then converts that force
into work.
A piston steam engine is either expanding or non-expanding, depending on whether the steam is
“cutoff” at some point in the piston travel or is admitted throughout the full stroke. Expanding engines
are proportionately less powerful because the pressure diminishes during the stroke, the exhaust steam
having very little available energy remaining to perform work. Non-expanding engines are
proportionately more powerful but much less efficient, the exhaust steam having much available energy
still remaining when leaving the cylinder. Because economy is an important aspect of automobile
engineering, we will confine our discussion to expanding engines.
Cutoff is expressed as a percentage of the stroke, if the valve closes ¼ of the way down the cylinder we
refer to it as a 25% cutoff. A more useful measurement is the expansion ratio, the ratio of the volume of
steam in an engine cylinder or turbine when the piston is at the end of the stroke to the volume at cut-
off.
The “clearance volume”, which is the volume between the top of the piston and the cylinder head when
the piston is at the most upwards position, must be established to determine the expansion ratio.
Given the engine cutoff of 25% (above), the change in volume in the cylinder will equal the volume
uncovered by the piston between the upper most point in its travel (called top dead center or TDC) and
the cutoff at 25% of the piston stroke. Let us also assume the clearance volume equals 25% of the
cylinder volume uncovered throughout a full piston stroke. The cylinder volume at cutoff is equal to
the clearance volume plus the cutoff volume. The total cylinder volume at the bottom of the stroke
(BDC) equals the volume uncovered by the piston plus the clearance volume.
The expansion ratio will be:
Total cylinder volume at BDC / cylinder volume at cutoff
The cylinder volume equals the volume at BDC, or 100% of the stroke volume, plus the 25% clearance
volume, or 125%. The volume at cutoff equals the cutoff of 25% plus the 25% clearance, or 50%.
125% / 50% = 2.5 to 1
Increasing steam expansion also tends to increase engine efficiency; less pressure leaves the engine by
way of the exhaust and is instead absorbed by the piston to do work. Eventually, the pressure drops to a
point where the work produced is less than the engine back pressure and friction, indicating very
practical limits to the amount of practical expansion.
Steam temperature also influences power and economy. Temperature falls along with pressure during
expansion; since saturated steam is at the condensation temperature, even a little expansion removes
enough heat to cause a portion of the steam to condense. Water occupies far less volume than the same
weight of steam and such condensation causes an accelerated drop in the cylinder pressure with an
accompanying fall off in work performed. Increasing the temperature above the saturation point
produces superheated steam which is able to expand further and produce more power before the onset
of condensation. The sensible heat required to produce superheat is small compared to the latent heat
of vaporization and thus the added work from superheating is significantly greater than the energy
used to add the superheat initially.
Steam enters at 500 psi in both cases, with a cutoff of 30% in the upper graph with a cut off at 30%
and 5% in the lower. The curves represent the pressure as the piston travels down the cylinder,
with the area beneath the curves being equal to the work developed. The average pressure for the
stroke in the upper case is 320 psi and 84 psi in the lower. We can say the Mean Effective Pressures
were 320 psi and 84 psi, respectively, and estimate that in the second case the engine is about
one-fourth as powerful according to PLAN.
“Mean Effective Pressure”, (MEP), the average steam pressure during an engine stroke, is proportional
to the power developed and generally inversely proportional to efficiency.
These graphs reflect the same engine running with the same steam pressure, but with using differing
cutoff:
Because 19^{th} century steam engine valves
usually admitted and exhausted steam
through the same port, the hot incoming
steam traversed a passage just travelled by
the outgoing cool exhaust, cooling the
incoming steam and causing premature
condensation, robbing efficiency. Breaking
the expansion into smaller steps reduces
the temperature drop in each cylinder, less
heat is transferred to the engine parts, leading
to a further efficiency gain. “Compounding” is
the process of breaking expansion into smaller
steps and to this day is the basis for our most
efficient and advanced steam and gas turbines.
Each expanding element is now termed a “stage”,
though at one time it was called an “expansion”;
thus an engine that expands the steam three
times is a triple expansion engine or a three-stage
expander. An expander with just one stage is
a “simple” expander and two stages a compound.
The drawing to the left illustrates the basic
components of a compound engine. The
smaller high pressure (HP) cylinder, to right,
partially expands steam which exhausts to a
receiver. The receiver levels out variations in
pressure and supplies steam to the larger
low-pressure cylinder (LP) which expands the
steam further. By adding stages, one can
accommodate higher steam pressures and
shorter cutoffs.
PLAN is an acronym for a formula to calculate theoretical horsepower in a single cylinder:
Pressure
(MEP, in psi)
Length
(of stroke, in feet)
Area
(cylinder inside diameter, square inches)
Number
(of revolutions, per minute)
Horsepower = (P x L x A x N) / 33,000
We can verify this equation by comparing it with basic terms in mechanics, the first being that 1
Horsepower = 33,000 foot-pounds per minute.
*
The Pressure (MEP) multiplied by the piston area determines the average force on the
piston in pounds.
*
The distance the piston travels in feet multiplied by the average force in pounds yields the
work produced in foot-pounds.
*
The work produced times the number of RPM calculates the power developed per minute
in foot-pounds per minute.
*
Dividing the power by 33,000 converts the work from foot-pounds per minute to
horsepower.
The area beneath the upper curve looks relatively ‘fat’ compared the relatively, ‘skinny’ lower
curve; engineers study such curves to determine both potential power and efficiency. Fat curves,
with their higher MEP, produce more horsepower for their size but do so by wastefully disposing
pressurized steam from the exhaust. Skinny curves indicate the steam is fully expanded and
operating efficiency but also indicates lower overall power output. Mechanisms called valve gears
regulate how early or late in the stroke cutoff occurs, adjustable valve gears can provide either
skinny or fat curves as needed.
Short cutoff implies the valve will be open briefly, which in turn requires high valve speed to
complete the cycle from closed to open and closed again in a short time; such fast operation is
technically demanding as extra stress, friction and wear must be managed. Overall efficiency
improves with the adoption of higher pressures and temperatures, if the engine can expand the
steam fully. The inability to use short cutoff practically limits useable pressures and efficiency. In
the 19^{th} century it became feasible to generate higher steam pressures, but remained a challenge to
build valves able to use the steam effectively. Suppose we desire a cutoff of 10%, but can only
practically build engines of 30%, it soon becomes apparent that the steam leaving the cylinder still
possesses enough pressure to operate another cylinder. By expanding transferring this exhaust
steam to a larger cylinder and cutting it off at 30% cutoff, transferring the steam to a larger
cylinder and expanding again with 30% cutoff, we achieve a higher overall expansion ratio than
our desired 10% cutoff. Rising pressures and temperatures led to the use of three and even four
cylinders.